Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Internal Combustion Engine Fundamentals

  1. Oct 8, 2005 #1
    In an SI engine, according to the book Internal Combustion Engine Fundamentals, knocking is produced due to the interaction of the shock wave and the expansion wave which together produce very high amplitude standing waves.

    My question is, in knocking is there only one shock wave or multiple shock waves?

    Would a single shock wave with a low pressure gradient (say 1 bar) be sufficient to damage the engine?
  2. jcsd
  3. Oct 11, 2005 #2


    User Avatar
    Science Advisor
    Gold Member

    I have never heard that knocking was caused by that. That's a poor explanation of knocking, which has its ultimate fundamentals in the Chemics involved.

    You shouldn't view knocking as a single detonative shock wave travelling inside the cylinder. Knocking is caused locally, in each element of mixture, and spreads a great amount of small spheroidal shocks which interact one each other. All of them gather togheter to form a pressure front.

    On the other hand, I doubt very much that a shock wave with such a low pressure gradient can exist. You should be able to work out how much damage can provoke. Just solve the pressure produced and calculate the stress exerted by the explosion in the piston rod, and compare it with the elastic limit of steel.
  4. Oct 11, 2005 #3


    User Avatar
    Gold Member

    Speaking as one who has blown a hole the size of a golf ball in a piston... ouch!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook